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Infectious moods: How bugs control your mind

By Linda Geddes

The brain is supposed to be isolated from the immune system – but now it seems that happiness, depression and even mental illness really can be catching

FEELING happy? Down in the dumps? Or been behaving strangely lately? Besides the obvious reasons, whether or not you are happy or sad, or prone to depression or other mental illnesses, could be a consequence of an infection – or even down to the diseases that you didn’t catch during childhood.

“It used to be thought that the immune system and the nervous system were worlds apart,” says John Bienenstock of McMaster University in Hamilton, Canada. Now it seems the immune system, and infections that stimulate it, can influence our moods, memory and ability to learn. Some strange behaviours, such as obsessive compulsive disorder, may be triggered by infections, and the immune system may even shape our basic personalities, such as how anxious or impulsive we are. The good news is that understanding these links between the brain and immune system could lead to new ways of treating all kinds of disorders, from depression to Tourette’s syndrome.

This is a massive shift in thinking. Not so long ago, the blood-brain barrier was thought to isolate the brain from the immune system. The cells that make up the walls of blood capillaries are joined together more tightly in the brain than elsewhere in the body, preventing proteins and cells getting into the brain. Now, though, it is becoming clear that antibodies, signalling molecules and even immune cells often get through, sometimes with radical effects. In fact, immune cells do not even need to …